This invention relates to the art of optical micro-electromechanical systems (MEMS) devices, and more particularly, to all-optical switching using MEMS devices.
One solution for all-optical switching employs two MEMS devices each containing an array of tiltable micro mirrors, e.g., small mirrors, which can reflect light, which herein refers to any radiation in the wavelength of interest, whether or not in the visible spectrum. An optical path is established for light supplied from an input source, e.g., an optical fiber, to an output, e.g., an output fiber, by steering the light using a first micro mirror on the first optical MEMS device, the first micro mirror being associated with the input fiber, onto a second micro mirror on the second optical MEMS device which is associated with the output fiber. The second micro mirror then steers the light into the output fiber. Each fiber connected to the system is considered a port of the system, the input fibers being the input ports and the output fibers being the output ports.
Often, the light to be steered from the input fiber onto the first micro mirror of the first optical MEMS device first passes through a micro lens that is associated therewith and is part of an input micro lens array. The function of each micro lens is to collimate the beam of light supplied from its respective associated input fiber. Alternatively, in lieu of employing a separate micro lens array, a respective lens may be integrated with each fiber of the fiber bundle in an arrangement that forms a collimator. Similar arrangements of micro lens arrays or integrated lenses are also found interposed between the output MEMS device and the output fiber bundle in the output section of the all-optical switch. In the output section, the function of each micro lens or collinator is to couple the light beam into its respective associated output fiber.
A problem in the art of all-optical switching using MEMS devices is that the center of any particular micro lens may not be lined up exactly with the center of its corresponding optical fiber. This causes the light beam to have a directional error, in that it does not travel directly toward the center of its associated micro mirror. If the distance between the micro lens and the MEMS device is large, which may be necessary to keep the input fiber bundle from blocking beams which are reflected from the micro mirrors of the MEMS device, the light beam will hit the micro mirror, if at all, off center. As a result, either no light will be reflected from the micro mirror if the beam does not hit the micro mirror at all, or the beam that is reflected will only represent part of the original beam, in that the part of the light beam that does not hit the micro mirror will be cut off, which results in attenuation of the light beam.
Similarly, in the output section, light which is reflected from an output micro mirror may not hit the micro lens, and as a result, will not be coupled into the output fiber. Alternatively, only part of the light may hit the micro lens, so that at most that part of the light could be coupled into the fiber. This results in attenuation of the light beam. Furthermore, even if the light hits the output micro lens, if the light comes in with an angle other than being parallel to the axis from the center of the micro lens to the fiber, then not all of the light reaching the micro lens will be coupled into the output fiber. Again, this results in attenuation of the light beam.
In other implementations of the all-optical switch, a micro lens array is not employed. Instead, each fiber has a lens integrated with it, to form a collimator, so that the light comes out as a parallel beam. While the fibers of the optical bundle may be made very regular, the direction in which the collimator lens is pointing may not be parallel to the line formed by the center of the lens and its associated micro mirror. This angle is often set by the angle of housing in which the collimators are mounted. If the angle of the lens is not parallel to the line formed by the center of the lens and its associated micro mirror, the light beam will have a directional error, in that it does not travel directly toward the center of its associated micro mirror. If the distance between the collimator and the MEMS device is large, which may be necessary to keep the input fiber bundle from blocking beams which are reflected from the micro mirrors of the MEMS device, the light beam will hit the micro mirror, if at all, off center. As a result, either no light will be reflected from the micro mirror if the beam does not hit the micro mirror at all, or the beam that is reflected will only represent part of the original beam, in that the part of the light beam that does not hit the micro mirror will be cut off, which results in attenuation of the light beam.
Similarly, in the output section, light which is reflected from an output micro mirror may not hit the collimator lens, and as a result, will not be coupled into the output fiber. Alternatively, only part of the light may hit the collimator lens, so that at most that part of the light could be coupled into the fiber. This results in attenuation of the light beam. Furthermore, even if the light hits the output micro lens, if the light comes in with an angle other than being parallel to the axis from the center of the micro lens to the fiber, then not all of the light reaching the micro lens will be coupled into the output fiber. Again, this results in attenuation of the light beam.
The same type of problem is manifest when using a wave guide in lieu of a fiber bundle.
Although It is easy enough to perform the alignment to insure that the light beam follows the desired path when there is only a single input fiber or a single output fiber. However, when there is a bundle of input or output fibersxe2x80x94which may include a thousand or more fibersxe2x80x94getting all the beams to be parallel is a very difficult task.
We have recognized that the foregoing problem of multiple light beams not being parallel and/or having an undesirable angle when they leave their source can be overcome by interposing an imaging system between the micro lens array and/or the collimators and the moveable micro mirrors of the MEMS device to which, or from which, the light beams are directed. Such an arrangement causes an image of the micro lens array and/or the collimators to be formed at the MEMS device, or vice-versa, due to the reversible nature of optics, thus effectively eliminating the distance between the micro lens array and/or the collimators and the MEMS device over which the light beams had previously traveled. Thus, advantageously, each light beam, even if not traveling in the desired direction parallel to the line formed by the center of its lens or collimator and its associated micro mirror does not get the opportunity to travel away from its intended target.
In one embodiment of the system, the imaging system reproduces the angle of reflection of the light from the first micro mirror, which may be achieved using a telecentric system, also known as a 4 f system. The physical size of the arrangement may be reduced by compacting the optical path, e.g., using appropriate conventional mirrors, and/or employing folded arrangements, i.e., arrangements in which there is only one MEMS device stage that does double duty for both input and output through the use of at least one conventional mirror. The overall system is arranged to account for any inversions introduced.